An erection means that your penis becomes stiff and stands erect. This happens when you are turned on. Sometimes it happens for no reason, especially during puberty.
How does it work?
Your penis consists largely of spongy tissue. When you are sexually aroused, this spongy tissue fills up with blood. This causes your penis to become hard, larger usually, and it stands erect. When you have a hard-on, your foreskin retracts and you can see your glans. Men who are circumcised can see their glans all the time, even when their penis is limp.
Oh dear... Your penis suddenly gets stiff when you're standing beside the swimming pool. Or when a doctor examines you. Or… Has it ever happened to you? It's all part of puberty. It's called a ‘spontaneous erection’. As you get older, it will go away automatically. What should you do when it happens? Just think about something else. Something that really doesn't turn you on. Your exams, what you're having for dinner tonight, your smelly socks that should have been in the wash 5 days ago...